Ultrasonic imaging tube

ABSTRACT

An improved ultrasonic imaging tube having a piezoelectric face plate or conversion plate, a controlling screen adjacent thereto and a collector electrode back of the screen for receiving current generated by the conversion plate. The collector current is utilized to control a video tube synchronized with the ultrasonic tube, and is shunted in feedback fashion to the screen to control terminal admittance thereof and thereby increase the signal to noise ratio of the system as well as to raise the effective signal level.

C United States Patent [1 1 [111 3,833,830 lurner Sept. .3, 1974 [5 ULTRASONIC IMAGING TUBE 2,957,340 10/1960 Rocha 313 641 x 3,013,170 12/1961 Sheldon 3l3/64.1 X [75] Invent: Turner, sliver Sprmg, 3,236,944 2 1966 Jacobs 3l3/64.1 x 3,505,558 4/1970 Jacobs 313 641 Assigneez Automation Industries, Inc, Silver 3,567,990 3/1971 Whymark 313/64.1 X

Spring, Md. Primary ExaminerRo bert Segal [22] Flled: June 13, 1972 Attorney, Agent, or FirmJones and Lockwood 21] Appl. No.: 262,418

Related US. Application Data J F [60] Continuation Of Ser. NO. 79 135 Oct. 8 1970 mpmved ultrasomc magmg tube havmg a abandoned which is a divisionof electric face plate or conversion plate, a controllmg Sept 3 1968, Pat 3,577,171 screen adjacent thereto and a collector electrode back of the screen for receiving current generated by the 5 21 as. o 313/369 conversion plate- The oollooror current is utilized to 51 Int. Cl. H0lj 31/495 control a video tube Synchronized with the ultrasonic [58] Field of Search 3l3/64.l lube, and is shunted ill feedback fashion to the Screen to control terminal admittance thereof and thereby in- 5 References Cited crease the signal to noise ratio of the system as well as to raise the effective signal level.

4 Claims, 5 Drawing Figures PAIENIEDSEP 3w" 3.833.830

FIG. 2

PRIOR ART INENTOR WILLIAM R. TURNER,

ULTRASONIC IMAGING TUBE This application is a continuation of my application, Ser. No. 79,135, filed Oct. 8, 1970, now abandoned which in turn is a division of my application Ser. No. 5 756,866, filed Sept. 3, 1968, now US. Pat. No. 3,577,171, issued on May 4, 1971.

This invention pertains to an improved ultrasonic tube which is equipped with a piezoelectric face plate or conversion plate capable of converting ultrasonic to electric signals and vice versa. The electric signals picked up by the ultrasonic tube can be displayed on a cathode ray tube acting as a video tube.

Heretofore, the signal current output from the high conductance ultrasonic converter tube hasbeen obtained directly from a screen closely spaced to the conversion plate. This screen, when swept by cathode ray beam, establishes the electrical field at the conversion plate surface necessary for the high conductance path between the electron beam landing point and the screen. However, the capacitance of the screen to ground sets a lower limit on the termination admittance, and hence, the signal level, achievable for a given bandwidth; the capacitance between the conversion plate surface and the screen introduces an extraneous signal current that must be removed by signal processing; and the small percentage of the primary electron beam intercepted by the screen produces a significant amount of tube noise.

The present invention avoids the defects of prior devices by the use of a high transparency screen and a separate collector electrode which draws the signal current therethrough. The electrode is so positioned at a distance from the screen that the capacity coupled signal and primary noise are eliminated. Moreover, the signal on the collector electrode is amplified in a circuit that balances out electrode capacitance and is then applied to the screen in feedback fashion to produce a potential which increases the electric field at the conversion plate. The effective termination admittance is thus reduced by the factor of current multiplication, increasing the degree of conversion plate element control and raising the output signal level.

Although the invention disclosed herein might be applied to various types of ultrasonic-electronic converters, optimum performance would be attained by utilizing it in the high-conductance type converter disclosed in prior US. Pat. to W. R. Turner, No. 2,903,617 of Sept. 8, 1959, or in the article entitled Ultrasonic Imaging by W. R. Turner published in Ultrasonics for October-December 1965, pp. 182-187. Also, it could be readily adapted to the Transceiver Ultrasonic Image System disclosed in the inventors co-pending US. patent application, Ser. No. 718,024 filed Apr. 1, 1968, now US. Pat. No. 3,600,936, which issued on Aug. 24, 1971, with beneficial effects accruing in its operation in reciprocal modes.

The principal object of the invention is to provide an improved ultrasonic-electronic video system having a higher signal conversion efficiency and reduced noise factor.

A further object of the invention is to provide an imaging tube equipped with a collector electrode in addition to the customary screen.

A still further object is to make an output connection from the aforesaid collector electrode to an amplifier, to feed said amplified signal to the video phase of a system, and to shunt said signal to the screen to lower the admittance margin of the imaging tube.

Still another object of the invention is to modify the geometry of an image tube so as to permit the use of a separate collector electrode independently of the screen adjoining the piezoelectric face of the tube, in the receiving mode of its operation.

With the above and related objects in view, the invention is described below in conjunction with the drawings, in which FIG. 1 is a partial diagrammatic showing of a high conductance image tube preceding this invention.

FIG. 2 is a partial analog circuit depicting properties of the tube in terms of electrical components, and including a coupling network utilized to balance out capacitive components thereof.

FIG. 3 is a partial cross-section of the image tube of this invention.

FIG. 4 is a diagrammatic circuit showing the several components and novel coupling employed in the output phase of the subject invention.

FIG. 5 is a diagrammatic representation of a video projection system to display the signal from the output of the tube of the invention.

Referring to fragmentary enlargement FIG. 1, the high conductance image tube disclosed in applicants US. Pat. No. 3,600,936 referred to above consists of an envelope, cathode ray gun and sweep deflecting plates (as in FIG. 3) and employs a piezoelectric face plate or conversion plate sealed to the envelope. The plate is convered by a grounded, thin, protective membrane generally designated 8 which is in contact with a suitable ultrasonic transmitting medium such as water. The other side of the plate 1 (within the envelope) carries an insulated target surface 3 which has a high secondary emission yield (greater than unity). Mounted in proximity of target surface 3, is a closely spaced but highly transparent wire capture screen 4. While the conversion plate 1 is continuous physically, it functions in cooperation with screen 4 and cathode ray beam or electron beam 2 as though it was composed of a plurality of discrete-transducer elements, the area 5 of which is determined by the area of the impinging beam. I

The impact of cathode ray beam 2 upon target surface 3, landing area 5, gives rise to a space charge 6 which serves as a coupling medium between landing area 5 and screen 4. When the space charge is in equilibrium, the electrons therein are at zero velocity and are free to move either to landing area 5 or toward screen 4, acting as a capture electrode. The direction of electron motion may be determined by minute changes of electric field in the vicinity of zero velocity position and random fluctuation in emission velocity. If an alternating potential should appear on landing area 5 due to piezoelectric action of plate 1, in response to ultrasonic signals 9, the space charge equilibrium will be disturbed and an alternating electron flow 7 therefrom will be driven to the screen; if, on the other hand, an alternating potential from a suitable external source (not shown) is applied to screen 4, the equilibrium of the space charge will be disturbed in the opposite direction, driving an alternative electron flow to landing space 5 and generating ultrasonic vibration on plate 1. The above describes the efiective, alternative operations of an imaging tube in receiving and transmitting modes, respectively. Thus, the electron beam 2 sweeping over target surface 3 forms a conductive (or resistive) link from the beam landing area to the screen 4 which provides for alternating current flow between them in opposite directions depending on operational mode.

FIG. 2 depicts, in dotted rectangle a, an analog of three discrete transducer elements (landing area) 5 of plate 1, which may be impinged by cathode ray beam 2 in a predetermined, desired sequence. Each such transducer comprises the following equivalent electrical components: a capacitance (C being the electrical capacitance of the element; a conductance 11 (6,) being the radiation conductance of the element when the front face of the conversion plate is coupled to an ultrasonic transmission medium 9 such as water; and a current generator 12 (i,) being the electrical analog of ultrasonic energy entering the element through the transmission medium. The result of such excitation is an alternating potential on the landing area. Alternately, an internal excitation of the element by an alternating current impressed upon the landing area will cause current to flow into the radiation admittance, and hence a conversion into ultrasonic energy that is projected into the transmission medium.

The relation between the equilibrium electron current and the potentials defining the electric field in front of the emission surface is given by,

where A is the space charge area, L, the spacing from the emission surface to the capture electrode, L the distance from the emission surface to the plane of minimum electron velocity, V the capture electrode potential, V the emission surface potential, and V the average secondary emission velocity in electron volts.

Altemately, the relation between the current and the potential difference from the beam landing point to the capture electrode can be defined as a small signal conductance,

in this expression, L being ignored in relation to L.

Thus, the action of the elctron beam can be represented as a conductive (or resistive) link from the beam landing point to the screen. This provides a bilateral path for alternating current flow between the beam landing point and the screen.

A negative capacitance network 16 is employed with the above high conductance image tube to improve performance. This is a class of network described in detail by H. W. Bode in his book Network Analysis and Feedback Amplifier Design. This network exhibits at its terminals a negative capacitance 17 (C,) in parallel with a termination conductance 18 (G The negative capacitance must be sustained over a frequency bandwidth sufficient to include the important sidebands of the electrical signal. It cancels the stray capacitances 14 and terminating on the screen, and under certain circumstances can also act through the beam conductance 13 to cancel the capacitance 10 of the beam landing point element only.

A necessary condition for cancellation of the element capacitance is,

where B, is the susceptance of the element capacitance 10 (C Since B is a small quantity, G, must also be limited in value if the required value of G, is to be realizable. 6,, however, is limited in minimum value by a network realizability relation derived by Bode and given approximately as,

GFC '4B where C is the total shunt capacitance to be cancelled at the image tube terminal, and B is the bandwidth of the network.

As a consequence of these relationships, a number of compromises have been necessary in selecting the exact geometry for the high conductance image tube of the prior art.

The beam conductance can be increased by reducing the spacing between the screen and the conversion plate surface. This, however, increases the capacitative coupling 14 between the conversion plate and the screen through which extraneous signal current flows from the remaining ultrasonically excited plate areas, and also increases the capacitance to ground for the screen, further limiting the minimum value of the termination conductance 13 (G Altemately, the beam conductance can be increased by increasing the primary beam current 2. Higher beam currents result in an increased spreading of the electron beam through space charge repulsion, and hence a lessening of electronic resolution. More important, however, is the noise associated with the small proportion of primary beam current intercepted by the screen 4. Since this beam noise current is a significant component of total tube noise, as beam current is increased, this resulting noise source can cancel the effectiveness of any signal current improvement.

The invention herein discloses a method of signal extraction and feedback that significantly changes the limitations on tube geometry, termination network character, and operating parameters so that improved image tube performance is possible. The fundamental departure from my prior imaging systems resides in the use of a separate collector electrode for signal current collection.

As will be seen in FIGv 3, the new imaging tube employs the usual conversion plate 1, having a grounded membrane 8 for contact with an ultrasonic conductive medium, and a secondary emission surface 3 impacted by cathode ray beam 2. The construction of the capture screen 4 is essentially unchanged from the prior high conductivity tube, except that its spacing from the conversion plate surface may be decreased. The current output of collector electrode 20 is located behind the screen. The electrode 20 is generally annular in configuration and may be formed as a conductive coating on the side wall of the image tube.

Signal current diversion to the electrode 20 is possible because the secondarily emitted electrons approach the screen 4 with a velocity corresponding to the potential difference between the landing point 5 and the screen 4. Thus, the high transparency of screen 4 directly captures only those electrons incident upon screen wires and a large fraction of the electrons are propelled beyond the screen and are returned to it only because the screen is set at a high bias potential with respect to all nearly grounded surfaces.

The change in electrostatic fields within the image tube to effect signal current diversion to the new electrode 20 is effected by reducing the DC bias potential on screen 4 to an intermediate value, and applying a higher potential to the electrode 20. The potential of screen 4 need only be sufficiently high with respect to grounded sections of the tube wall to cause it to act as a control and to assure that all secondarily emitted electrons 7 will pass through the screen rather than terminate upon grounded areas adjacent to the emission surface. The bias potential on the electrode 20 must then be sufficient to divert the electrons to electrode 20 before axial velocity has carried the electrons beyond the electrode 20 to other portions of the tube.

it should be noted that the diversion of signal current from the screen to a separate output electrode has no effect on the screens control of emission at the space charge layer. Current released from the space charge region is wholly controlled by the potential difference between landing point 5 and screen 4. This will not be affected by bias changes on the screen because the insulated surface of the conversion plate will automatically adjust itself with reference to the screen to the potential difference given by,

Current multiplication with capacitance compensation is obtained using a differential voltage amplifier 22. The terminal 21 of collector electrode 20 of the image tube is connected directly to one terminal of the amplifier, with precautions to limit wiring capacitance. Capacitance 23 (C,,) is the total capacitance associated with the electrode, and conductance 24 (G is the total conductance. The amplifier is selected for high input impedance, low shunt input capacitance, and low equivalent input noise. lts output is fed back to screen 4 from junction 30 via terminal 19.

The signal for the inverse phase to the amplifier is a voltage derived from a current transformer 25 shunted by capacitance 26 (C and conductance 27 (G Assuming a unity ratio transformer, the ratio of current is given by the equation,

where Y, is the total termination admittance at the screen terminal 19 and A is amplifier gain. The second expression results if the product Y A is made large compared to (G -l-jwc A further simplification is possible if the ratio G /wC is made equal to G /wC i /l CJCKK Thus, the current i,, is multiplied by K, and this multiplication factor is set by a capacitance ratio, one of the capacitances being the termination capacitance for the electrode.

For state-of-the-art operational amplifiers suitable for this application, the shunt conductance at the electrode terminal will be small compared to the shunt capacitance. The effect of this conductance, however, can also be compensated by the adjustment of the G Two signal outputs A and B are indicated in the block diagram. Output A is through the negative capacitance network 16 connected to the junction 30. An output of instability for this application is capacitative coupling between screen 4 and the output electrode 20. An electrostatic shield 29 (FIG. 3) for the lead from screen 4 to terminal 19 is shown as being indicative of methods by which this coupling can be reduced. Other forms of electrostatic shielding or neutralization techniques might be used.

The alternating potential applied to the screen is given by,

where K is the fraction of the original secondary signal current passing through the screen and becoming i,,. The effective termination admittance at the screen is,

Thus, the effective termination is the actual admittance of the screen to ground including stray capacitance and the effect of the termination network, divided. by the net current multiplication. The small fraction of i,, terminating on the screen is ignored in this analysis as being insignificant in relation to i Similarly, the current arising from capacitative coupling to the screen, and the noise current intercepted from the primary beam are regarded as insignificant in comparison to i if a large value is used for G, and A.

. New limits, of course, will exist on how large K can be made, how large G, can become, and how small C,, can be made, etc. Nevertheless, within these limits a major signal-to-noise improvement will be possible.

Having described the invention with reference to a preferred embodiment, it is to be understood that mod- 7 8 ifications may be made for practicing the invention said discrete area of beam impact to produce secwithout departing from the spirit and scope thereof. ondary emission over the surface of said conver- I claim: sion plate; A low noise, high Conductance ultrasonic g g means including said control screen and the said distube, p g: crete area of said conversion plate at the point of an envelope; beam impact for applying an alternating electrical a Conversion Plate Capable of Converting ultrasonic potential to said space charge region whereby there vibrations to and from electrical signals sealed to is a fl f Secondary electrons between id P h of the ehvelope ahd formihg the face of the version plate and said control screen through said hhagmg tube; I 10 space charge region; and a grounded membrane on the outer means including said discrete area of said conversion F of f com/5x510 plate; plate at the point of beam impact, said control an msulatnjlg. coatmg havmg .secondary. elmsslon screen, and said annular collector electrode directg f zg gzgiziggg ig zh on the mslde 3 ing secondary electrons emitted by said coating,

p l5 and flowing through said space charge region toa cathode ray gun within said envelope and located at the end opposite said conversion plate to generate a cathode ray beam;

a control screen of high electronic transparency mounted within said envelope in the path of said beam and in close proximity and parallel to said conversion plate to reduce the capacitive coupling between said collector electrode and said conversion plate;

an annular collector electrode located within said envelope adjacent said control screen and concentric with the longitudinal axis of the cathode ray tube so as not to intercept current from the beam;

means directing said beam toward said conversion area of 531d Conversion P plate, said beam passing through said control The g g tube of Claim further including screen and impacting on a discrete area f id electrostatic shielding between said annular collector conversion plate coating to pr d i i f electrode and the leads attached to said control screen,

ward said screen in response to ultrasonic vibrations received by said conversion plate, through said electronically transparent control screen to said annular collector electrode to form an output current thereon that is substantially free of beam noise current.

2. The imaging tube of claim 1, wherein said collector electrode is a conductive coating on the inner surface of said envelope.

3. The imaging tube of claim 1, wherein said means for applying an alternating potential to said space charge comprises piezoelectric action at said discrete secondary electrons from said coating and to prothereby reducing substantially the capacitive coupling duce a space charge region between said screen between said collector electrode and said control and said plate; screen.

means for sweeping said cathode ray beam to move 

1. A low noise, high conductance ultrasonic imaging tube, comprising: an envelope; a conversion plate capable of converting ultrasonic vibrations to and from electrical signals sealed to one end of the envelope and forming the face of the imaging tube; a grounded conductive membrane on the outer surface of said conversion plate; an insulating coating having secondary emission characteristics greater than unity on the inside surface of said conversion plate; a cathode ray gun within said envelope and located at the end opposite said conversion plate to generate a cathode ray beam; a control screen of high electronic transparency mounted within said envelope in the path of said beam and in close proximity and parallel to said conversion plate to reduce the capacitive coupling between said collector electrode and said conversion plate; an annular collector electrode located within said envelope adjacent said control screen and concentric with the longitudinal axis of the cathode ray tube so as not to intercept current from the beam; means directing said beam toward said conversion plate, said beam passing through said control screen and impacting on a discrete area of said conversion plate coating to produce emission of secondary electrons from said coating and to produce a space charge region between said screen and said plate; means for sweeping said cathode ray beam to move said discrete area of beam impact to produce secondary emission over the surface of said conversion plate; means including said control screen and the said discrete area of said conversion plate at the point of beam impact for applying an alternating electrical potential to said space charge region whereby there is a flow of secondary electrons between said conversion plate and said control screen through said space charge region; and means including said discrete area of said conversion plate at the point of beam impact, said control screen, and said annular collector electrode directing secondary electrons emitted by said coating, and flowing through said space charge region toward said screen in response to ultrasonic vibrations received by said conversion plate, through said electronically transparent control screen to said annular collector electrode to form an output current thereon that is substantially free of beam noise current.
 2. The imaging tube of claim 1, wherein said collector electrode is a conductive coating on the inner surface of said envelope.
 3. The imaging tube of claim 1, wherein said means for applying an alternating potential to said space charge comprises piezoelectric action at said discrete area of said conversion plate.
 4. The imaging tube of claim 1, further including electrostatic shielding between said annular collector electrode and the leads attachEd to said control screen, thereby reducing substantially the capacitive coupling between said collector electrode and said control screen. 